Note: Descriptions are shown in the official language in which they were submitted.
97
SPECIFICATION
P,ACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to an induction heating
appara-tus for heating cooking ware and thereby cooking food,
wherein a transistor is employed as a main control element.
Description of the Prior Art
A thyristor inverter or a transistor inverter has
heretofore been employed as a high frequency power source for
an induction heating apparatus which embodies the principle
of induction heating wherein a magnetic body itsel~ is caused
to generate by -the eddy current induced on the crossing of a
magnetic field of a coil with the magnetic body. As methods
for controlling ~he output of such a device, it may be
contemplated (a) to vary the DC source vol-tage of -the inver-ter
circuit, (b) to vary the duty cycle of the inverter circuit or
(c) to resort to an on-and-off duty control of a power switching
semiconductor. However, these methods have their own
disadvantages. Thus, the method (a) is expensive, the method
(b) has the drawbacks of RAMP flicker and delayed heating
response, and -the method (c) tends -to entail an overloading of
the power switching semiconductor and, thereforel demands a
power switching semiconductor with a high breakdown voltage
and a high capacity. Especially in the case of a -thyristor
inver-ter, a decrease of inverter oscillation frequency results
_i_
in a decrease of output and, conversely, a higher oscillation
frequency means an increased output. To reduce the losses of
the power switching semiconductor design-wise, the oscillation
frequency may be legitimately reduced, but lf the hea-ting output
is to be decreased by changing the frequency, the la-tter of
necessity will fall within the audible range. If an ultransonic
frequency range is maintained at a low outpu-t, a large current,
a high voltage resistance and increased switching losses are
inevitable and therefore, output control over a broad range
cannot be accomplished. If a broad range of output control is
sought with a transistor inverter, a method has had -to be
devised which would permit output control over a broad range
without overloading the transistor.
OBJECTS AND SUMMARY OF THE INVENTION
The above problems have been solved by -the present
inven-tion. In accordance with the invention, output control
is effected by varying the continuous on-off ratio of a
-transistor without adversely affec-ting the condi-tion of the
power switching semiconductor. Thus, the presen-t inven-tion
is characterized in that as the turn-on pulse width of the
transis-tor is increased, the heating ou-tput increases, while
a narrowed trun-on pulse width results in a reduced heating
output and that as the heating output is decreased, both the
current and voltage of the -transistor decrease and the
oscillation frequency becomes higher.
~ l66~f~7
Ano-ther advantage of this invention is that it provides
for output control wit~h a continuous and very rapid response
and that the input curren-t, i.e. input power, can be controlled
at any level ~hat may be desired by the user. The desired
heating output can be obtained with a rapid response at a level
corresponding to the product of input power multipli-d by an
efficiency of about 80 percent. For example, smooth continuous
output control can be achieved over the range of 50W through
1500W. The apparatus according to the present invention is
free from flicker and can be constructed at low cost.-
Another advantageous feature of the present inventionis that the input current of an inverter circuit and the
collector voltage of a transistor are detected and feedback
control is applied so as to ensure a stable operation
irrespective of loads.
Since, in accordance with the present invention,
control of the turn-on pulse width of the transistor over a
broad range is accomplished by a pulse width modifica-tion
circuit equipped with a synchronously oscillating RA~P generator,
pulse width control over a broad range can be accomplished.
The abovenoted object may be effected by providing an
induction heating apparatus comprising an inverter circui-t
for converting a DC electric power to a high frequency electric
power and a control circuitry associated therewith, said
inverter circuit comprising a heating coil, a power semiconductor
block consisting of a power switching semiconductor connected
in series circuit relation to said heating coil and a damper
;" ;
,., ,~
''37
diode connected in reverse parallel circuit relation -thereto and
a resonance capacitor forming a resonance circui-t with said
heating coil, said control circuitry including a voltage
detection means for detecting the voltage oE said heating coil
and a pulse width modification means for controlling the turn-on
pulse width of said power switching semiconductor in response
to the output signal of said voltage detection means.
The object may-also be e-ffected by providing an
induction heating apparatus as above, wherein said control
circuit includes a means for detecting the input power of
said inverter circuit and an error amplifier for comparing
the output signal of said input power detection means with the
level set by a user, thereby controlling the -turn-on pulse
wid-th of said power switching semiconductor and -the heating
output to a desired level, or by providing an induction
heating apparatus as above, wherein said heating coil voltage
detection means detects a substantially zero point of said
heating coil voltage to control the turn-on pulse width of said
power switching semiconductor.
The object may further be effected by providing
an induction heating apparatus as above, wherein said heating
coil voltage detec-tion means is a comparator adapted to compare
the input DC voltage of said inverter circuit with the voltage
of said power switching semiconductor, and/or providing an
induction heating apparatus whereln said control circui-try
includes a detection means for detecting the input power of
said inverter circuit, a detection means for detecting the
vol-tage of said power switching semiconductor, respective level
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i297
setting means and error amplifier means therefor and OR circuit
means respectively connected to the outputs of said error
amplifiers, whereby whichever one of said detection means that
produces an output signal exceeding the correspondlng se-t level
is given priority.
The object may s-till further be effected by providing
an induction heating apparatus wherein said control circuitry
includes a voltage comparator for detecting the timing a-t which
the voltage of said heating coil undergoes a transition from
nega-tive sign to positive sign or at which -the voltage of said
power switching semiconductor becomes less than the input
voltage of said inverter circuit, a synchronizing pulse generator
means, and a pulse width modification circuit means for
controlling the turn-on pulse width of said power switching
semiconductor in synchronism with the output signal of said
synchronizing pulse generator means, or by providing an induction
heating apparatus wherein said control circuitry includes
limiter means for set-ting a maximum and a minimum limit oE the
turn-on pulse width of said power switching semiconductor
connected to said OR circuit means and a start-stop control
circuit for controlling the oscillating operation of said
inverter circuit, said start-stop control circuit having a
soft start means for causing each start to take place from the
minimum pulse width set by said limiter means.
The object may also still further be effected by
providing an induction heating apparatus comprising an inverter
circuit for converting a DC electric power to a high frequency
electric power and a control circui-try associa-ted therewith,
said inverter circuit including a heating coil, a power
semiconductor block consisting of a power swi-tching semiconductor
connected in series circui-t relation to said heating coil and
a damper diode connected in reverse parallel circuit rela-tion
thereto and a resonance capacitor forming a resonance circui-t
with said heating coil, said control circuitry including a pulse
width modification circuit for controlling the turn-on pulse
width of said power switching semiconudctor in synchronism with
a circuit parameter of said inverter circuit, a base driver
circuit, an electric signal detection means for detecting the
electric signal corresponding to the input or output level of
said inverter circuit, and user control means for selecting a
desired output level, wherein said inverter circuit is oscillated,
at low output, in feed forward mode where said damper diode is
not turned on and, at high output, in quasi-E class mode where
said power switching semiconductor is turned on after said
damper diode has been turned on, or by providing an induction
heating apparatus as above, wherein the heating appara-tus is
conti.nually controlled over a heating output range of 50W to
1500W by control of the turn-on pulse width of said power
switching semiconductor block.
This invention will hereinafter be described in detail
by way of the preferred embodiments which are illustrated in
the accompanying drawings.
37
BRIEF DESCRIPTION OF T~E DRAWINGS
Fig. 1 is a block diagram showing an induction heating
apparatus as an embodiment of this invention;
Fig. 2 is a circuit diagram showing a control circui-t
for the same apparatus;
Fig. 3 is a diagram showing the waveforms of the same
apparatus at high output;
Fig. 4 is a diagram showing the waveforms of the same
apparatus at low output;
Fig. 5 is a waveform diagram of the transistor voltage
VcE and heating coil voltage VL of the same apparatus;
Fig. 6 is a diagram showing the waveforms modulated by
an alternating source voltage; and
Fig. 7 is a circuit diagram of another detection
circuit of the same apparatus.
DETAILED DESCRIPTION OF THE INVENTIOM
Referring to Fig. 1, an alternating current from a
low frequency AC power source 1 is applied to a rectifier circuit
2, whereby it is converted to a DC power. The DC power output
of said rectifier circuit 2 is fed to an inverter circuit 3,
where it is converted to a high fre~uency power. The inverter
circuit 3 comprises a choke coil 30 and an input capacitor 31
connected in series and a series circuit consisting of a heating
coil 32 and a transistor 33 which is a power switching semiconductor
in parallel circuit relationship with the input capacitor 31.
Connected to the transistor 33 in reverse parallel circuit
..
relationship is a damper diode 34 whi.ch is arranged so as to
constitute a power switching semiconductor block. An oscillation
capacitor 35 is connected to the heating coil 32 in parallel
circuit relationship. The same resul-t is ob-tained when the
oscillation capacitor 35 is connected to the transistor 33.
The construction of a control circuit 4 is described
below. The terminal voltage VDc of an input capaci-tor 31 and
the collector voltage VcE of transistor 33 are applied to a
voltage compara-tor circuit 40, a synchroniza-tion circuit 41
detects a point where the collector voltage VcE is lower than the
input terminal voltage VDc and a pulse width modifying circuit
(briefly, a PWM circuit~ 42 generates a base drive signal Eor
transistor 33. The output signal of PW~ circuit 42 is applied
to a driver circuit 44 via a gate-prohibition circuit 43. The
driver circuit 44 provides a forward base current IBl and a
reverse base current IB2 to the transistor 33~ The PWM circuit
42 controls the turn-on pulse width of the transistor 33
according to a circuit signal corresponding to the input and
output levels of inverter circuit 3, and a current transformer
45 detects the input curren-t of inverter circui-t 3 and feeds
it to an input detector circuit 46 which transforms the input
signal into a voltage signal corresponding there-to. The output
signal of said input detector circuit 46 is fed to a first error
amplifier 47 which performs amplification in comparison with an
output signal of a user setting means 48. The output signal
of said first error amplifier 47 is fed to an OR circuit 49.
On the other hand, the transistor collector voltage VcE is
~~ J~1$~
applied to a collector voltage detector circuit 50 which de-tects
the collector voltage VcE or a peak voltage thereof Vcp and
applies it to a second error amplifier 51. The error signal
obtained with respect to the user set-ting circui-t 52 is
amplified and supplied to the O~ circuit 49. The OR circuit is
such that the minimum or maximum of multiple input signals is
given priority and when either the input current or the collector
voltage VcE exceeds a set level, one of the signals is given
priority. The output signal of the OR circuit 49 is applied to
the PWM circuit via a limiter circuit 53. The limiter circuit
53 regula-tes the minimum and maximum values of the turn~on pulse
width of said transistor 33. The starting and stopping of the
oscillation of the inverter circuit 3 are both controlled by a
start-stop circuit 54, and the pulse width modi:Eication signal is
controlled by a signal to the gate prohibition circuit 43.
At the time of oscillation startup, a pulse width modification
signal is driven at the minimum width.
In Fig. 2 is illustrated a specific example of the
control circuit according to this invention. A voltage
comparator circuit 40 applies a signal vol-tage to one of -the
input terminals of a comparator 400 which has been obtained by
dividing an input DC voltage VDc by resistances 401a and 401b,
and applies a signal to the other input terminal which has been
obtained by dividing a collector voltage VcE by resistance 402a
and 402b. Fig. 3 shows the collector voltage VcE, input DC
voltage VDc and -the output signal Vc of voltage comparator
circuit 40. A compulsory synchronization circuit ~1 detec-ts
~ ~ _g_ -
2~37
the rise of signal Vc by means of a differen-tiating circuit
consisting of a differentiating capacitor 410 and a differen-tia-ting
resistor 411, and by utilizing -the threshold voltaye of inverter
412, causes a diode 413 on the output side of inverter 412 -to
generate a trigger signal Vt. The P~M circuit 42 consists of a
RAMP generator and a comparator, said RAMP generator being a
compulsorily synchronizable self-exciting oscillation circuit.
The ~ input setting voltage of an open collector comparator
420 is varied by the on-and-off operation of its output
transistor by means of a potential divider circuit of
resistances 421a and 421b and a potential divider circuit of
resistances 422a and 422b. When the output transistor of
comparator 420 is off, a charging circuit comprising a resistance
422a and a charging resistance 423a is driven, and when said
output transistor is on, a charging resistance 423b and a diode
424 charge and discharge a timer capacitor 425 -to produce a
RAMP waveform Vr. The RAMP waveform Vr and the pulse wid-th
setting signal Vs are applied to a comparator 426 -to make a
pulse width modification signal Vp. AS a trigger signal V-t
compulsority depresses the set input for comparator 420 oE the
RAMP generator, the timer capacitor 425 is discharged rapidly.
By this discharging circuit, the pulse width modification signal
is delayed with respect to the output signal of voltage compara-tor
circuit 40. The trigger signal Vt causes a compulsory
synchronization to produce a RAMP waveform synchronized with the
collector voltage Vc~. As the pulse width setting voltage Vs is
increased, the pulse width tl of the pulse width modification
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g~
signal Vp is broadened and the ou-tput increased. The signal Vp
is applied to the drive circuit 44 via the gate-prohibition
circuit ~3. During the period tl, a forward base curren-t X~l is
fed from driver circuit 44 to transistor 33 and then, is applied
as a reverse bias voltage between the base and emitter of
transistor 33, a reverse base current flows. The forward base
current IBl is delayed by ~t from the point to where VcE is equal
to VDc, and is applied at substantially the same timing as -the
turn-on time of damper diode 34. Since the base current is
supplied before the collector curren-t IC begins to flow, the
effect of transistor 33 is substantially eliminated. On the
turn-off of the transistor 33, the collector voltage VcE rises
slowly and sinusoidally so that the turn-off switching loss is
reduced to a minimum. The current IL of heating coil 32
approximates a sine wave.
The output signal of current transformer 45 is fed to
an input detection circuit 46, whereby it is converted to an
output signal substantially proportional to the input current.
The input detection circuit 46 is a filter circuit consisting
of a rectifier circuit 460, a discharge resistance 461 and an
integrating capacitor. The first error amplifier circui-t 47
is an inversion amplification circuit using an ordinary
operational amplifier and its input current can be set to an
optional Level by means of a setting circuit consisting of a
resistance 480 and a variable resistance 481, i.e. a user
control means. Priority is given selectively to one of the
outputs of sai~ first and second error amplifiers 47 and 52 by
3Z~7
an OR circuit means 49 comprislng OR connected diodes 490, 491
and 492. In this embodiment, priority is given to the lowest
of the output levels of error amplifier circuits. At an
oscillation start, a soft start signal sets the diode 492 at
a low level and reduces the pulse width setting signal Vs to
the lowest level which is limited by limiter means so as to
cause starting from the narrow pulse width. The limiter circuit
53 confines the voltage of capacitor 530 witnin the upper and
lower limits which are defined by the divisional potential
provided by resistances 531a and 531b, a transistor 532 and the
divisional potential provided by resistances 533a and 533b which
set the minimum volta~e. The control circuit for collector
voltage VcE is substantially identical with that for the input
current. The collector voltage VcE gains when the input DC
voltage VDc is increased or when the cooking ware load is of a
certain type (e.g. cooking ware of aluminum, nonmagnatic
stainless steel or cast iron) and, therefore, a protective
function is required.
Fig. 4 shows the waveforms at low output. Here, the
operation mode of the inverter circuit 3 is a feed forward
mode. The mode shown in Fig. 3 is a ~uasi-E class mode where
the -turn-on loss of the transistor 33 is substan-tially nil.
In contrast, the turn-on loss is large in a feed forward mode.
Thus, in a feed forward mode, the damper diode 34 is not turned
on. This is because as the turn-on pulse width of the transistor
33 is narrowed, the electromagnetic energy accumulated in the
heating coil during the turn-on period of the transistor 33 is
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~J~ 6¢~
throughly consumed by the load during the turn-off period of -the
transistor 33 so that no energy will be available for applica-tion
to the DC power source. Thus, the collector voltage VcE of the
transistor 33 is not zeroed; i.e. the voltage of heating coil
32 does not increase beyond the DC voltage VDc, with the result
that the damper diode 34 is not turned on. The transistor 33 is
turned on when its collector voltage VcE is o-f positive sign
and, therefore, the collector current Ic has an impulse of
peak current Ip. A turn~on loss occurs at this moment. However,
since the collector current and voltage are both low during the
turn-off time, the turn-off loss is very small. The turn-on
loss is also not so large. The overall switching loss, therefore,
is less than at the maximum output.
Fig. 5 shows the collector voltage VcE and the
heating coil voltage VL, where VDc = VcE ~ VL. Thus, VL
V~C ~ VcE. ~herefore, the point where the heating coil voltage
VL is zero is VDc = VcE. In other words, comparison of inpu-t
DC voltage VDc with collector voltage VcE is equivalent to the
detection of the zero crossing point of heatlng coil voltage
VL.
Fig. 6 shows -the manner in which the VcE and Vc
envelopes change actually. It will be apparent that a zero
crossing point of VL or a point where VcE is equal to VDc
exists for any load and any input waveform.
In Fig. 7 there is illustrated ano-ther version of the
voltage comparator circult 40 according to the present invention.
In this embodiment, the emitter of a high breakdown voltage PNP
r .: --13--
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37
transistor 403 is connec-ted to the inpu-t DC side of the circuit
and the base of said PNP transistor 403 is connected to the
collector voltage VcE side of the circ~lit via resis-tance 404.
A diode 405 is connected to the base and emitter of transistor
403 in a reverse parallel circuit relationship. Between -the
collector of PNP transistor 403 and ground -there are disposed
resistances 406 and ~07 arranged in a series circuit relationship,
and a zero crossing detection is erfected by means of the vol-tage
across resistance 407.
Thus, in accordance with the present invention, outpu-t
control over a broad range is accomplished by means o-f an
inverter unit which oscillates in a quasi-E-class mode a-t high
output and in feed forward mode at low output.
The novel and advantageous features of the present
inven-tion may be summarized as follows:
(1) Stable oscillation is ensured in a synchronized
relationship with the zero crossing detection of the heating
coil voltage~
~ 2) ~ synchronizable RAMP generator permits control of
transistor pulse width over a broad range, thus broadening the
available outpu-t control range.
(3) The detection and OR control of input current and
collector voltage provide for stable operation under all load
conditions without overloading the output transistor.
(4) There is a soft start feature and a s-table turn-on
performance is assured.
(5) The input power, that is, the heating outpu-t, can
be continually varied and controlled at any desired level~
`~ -14-
without RAMP flickers.
(6) Quick output control is assured, and ou-tput control
does not overload the outpu-t transistor.
(7) Because only the input voltage VDc and collector
voltage VcE need be compared, the required de-tect.ion and con-tro3.
circuits are simple.
(8~ The small switching loss and hi.gh efficiency of
the output transistor permit a small lightweight hardware design.
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